Prey - Douglas Hill

Nick Walker can pass for eighteen, which comes in useful while living on the streets. But Nick isn't eighteen. He's fourteen. He's been fourteen for years . . .

A chance encounter with Manta, an exceedingly powerful witch, sees Nick cursed with changelessness. Not only will he never age, but, more usefully to Manta, broken bones and sliced flesh, while excruciatingly painful, will mend themselves in a short space of time - making Nick the perfect weapon against the demonic assassins sent by the evil Cartel. Constantly hunted, Nick is forced to run - evading his enemy when he can and facing gruesome and bloody battles when he can't.

But, help comes from a surprising source - April, a young girl Nick rescues from some occult enthusiasts intent on sacrificing her, has powers beyond her control or understanding. Together, might they be a match for the sinister forces that stalk them?